Peripherals may be connected to a host computer using a USB (Universal Serial Bus). Moreover, peripherals may be connected to a computer through a bus powered USB hub. Bus-powered USB hubs are “high power” USB devices. This means that the total power drawn by the hub itself, together with all downstream devices is limited to 500 mA at the high end.
Many laptops have two USB (Universal Serial Bus) ports that may supply current to downstream sources. These USB ports are configured to each supply 500 mA of operating current to downstream devices. Most conventional mass storage devices require 0.5–1.0A of operating current for operation purposes. Because of this requirement, the limited levels of current that can be supplied from a conventional USB bus are not sufficient to power devices that require more than 500 mA of operating current. Consequently, a typical requirement of such devices is that they be “mains powered”, e.g., powered by an AC outlet.
A typical 2.5″ HDD (hard disk drive) requires in the range of 0.5–1.0A, at 5 volts to operate. In some cases, the power requirements of CDRW and DVD-ROM drives are similar. However, 5 volt only CDRW and DVD-ROM drives are less common, as these drives typically require both 5 and 12 volts to operate. By contrast, a typical USB-ATAPI (AT attachment packet interface) bridge IC device requires 200–300 mA of operating current at 3.3 volts for operation purposes. Although in time it is likely that the power consumption requirements of CDRW and DVD-ROM drives may fall to a level where 500 mA operation is common, there is an immediate demand for wholly “bus-powered” devices, e.g., mass storage devices of the aforementioned types that today require much higher levels of current for operation purposes than is available using conventional USB architecture. It would be advantageous if USB based devices were positioned to meet this demand.
To meet the current demand for wholly bus powered mass storage devices and other type devices, usage models must be developed that allow such devices to be powered by relying solely on the power supplied from USB host devices through conventional USB ports. This should be accomplished despite the limited levels of current conventionally provided by USB host devices through such ports.
A typical usage model provides that one of the ports of a USB host device, such as a laptop, be used to connect a system device, such as a mouse, and the other port be used to connect other peripherals such as USB cameras, PDAs, and the various types of mass storage devices. A disadvantage of this conventional architecture is the typical requirement that the peripheral be “mains powered”. This requirement is not practical for most mobile usage models because the remote availability of such sources of power may not exist. In addition, where rechargeable batteries are used to provide the required additional power, significant additions to the cost, weight and bulk of the peripheral package consequently result. Moreover, when mass storage devices having lower power requirements do become available, they are likely to be substantially more costly than are the currently available drives.